Most of my foamies are made of foamboard, so I'll assume the GWS planes are using similar foam. Water based acrylic paint from your LHS is fine. Can be brushed or sprayed (w/o a solvant). Just be careful that you're not putting it on too thick, it adds weight quickly.

Krylon Short Cuts also work, but the propelland melts the foam, so you need to put it on lightly, and not too close, to let the solvant evaporate quickly.